This invention relates to a multiplex control system which can be used to vary the power applied to a load such as an automotive headlamp in order to control the intensity of the headlamp.
It would be desirable in modern automotive vehicles to run the headlamps, and in particular the high beam headlamps, of the vehicle at reduced intensity during daytime vehicle operation. By operating the high beam headlamps at reduced intensity, the vehicle is made more visible to oncoming drivers. High beams operated at full intensity can be annoying to other traffic, and by operating the high beams at reduced intensity the life of the lamp is markedly increased.
It has been suggested that such reduced intensity operation can be provided by connecting the headlamp in series with a power resistor to reduce the voltage applied to the lamp on a continuous basis during daylight operation. This approach provides the disadvantages of increased power drain on the electrical system of the vehicle and the heat and expense associated with suitable power resistors.
For some time there has been an interest in applying multiplex systems to control the components of vehicles such as automobiles. Britain U.S. Pat. No. 4,293,947 is one example of such a multiplex system. In spite of this interest, currently available vehicle multiplex systems do not fully exploit the potential of such systems for providing improved control functions in vehicles.
Another, previously unrelated set of patents relates to the use of pulse width modulation techniques to control the intensity of fluorescent lamps and incandescent lamps. Stroede U.S. Pat. No. 4,570,108 discloses a control system for an incandescent lamp which varies the duty cycle with which power is applied to the lamp in order to reduce maximum current in the lamp during start-up conditions and to dim the lamp. Similarly, Kane U.S. Pat. No. 4,464,606 and Tarroux U.S. Pat. No. 4,398,131 use pulse width modulation techniques to vary the intensity of lamps. None of these patents relates to multiplex systems, and all require special purpose, complicated, and relatively expensive circuitry to accomplish the desired pulse width modulation.